In many mobile radio standards, provision is made for the level of the output signal to be altered in order to reduce influences on neighboring subscribers. This requires alteration of a gain factor for the amplifier provided for amplifying the output signal. In addition, a wide bandwidth and also a high level of linearity are demanded in order to meet the needs for increasing data transmission rates. These requirements relate both to the reception path, in which it is necessary to achieve a good signal-to-noise ratio, in particular, and to the transmission path, where the smallest possible distortions in the output signal are of particular importance.
One great problem of all amplifiers implemented to date using complementary MOS technology for mobile radio systems is a large capacitive component of an input impedance. The capacitive component makes matching firmly prescribed source impedances, for example from 100 to 200 ohms for the GSM or the UMTS/WCDMA mobile radio standard, a particularly complex matter. In addition, the filter elements used in mobile radio systems today have the side effect that they convert differentially rejected signals into the DC signal components. In the amplifiers connected downstream of the filters, the DC signal components cause an unwanted compression response and increase data transmission errors as a result of the arising nonlinearities in the useful signal.
It is therefore expedient to produce the greatest possible DC signal component rejection in the amplifier, this also being referred to as “common mode rejection”.
One method which is used frequently for compensating for the large capacitive component of the input impedance is inductive coupling. However, a particular problem in this context is a large surface area taken up by the integrated circuit in a semiconductor chip on account of the surface area intensive integration of coils on a silicon base.
An amplifier arrangement which reduces the surface area taken up is known, by way of example, from the German patent application 10 2004 025 918.6 submitted by the applicant.
However, an input impedance of a stage connected downstream of the amplifier arrangement has a significant influence on the input impedance of the amplifier arrangement. Changes in the input impedance of the downstream circuit can therefore require total redesign of the amplifier disclosed in the aforementioned application.